In recent years, it has been desirable to employ aluminum nitride (AlN) substrates for circuit boards used under high-temperature environments, particularly for high power applications. AlN substrates have been promising candidates due to their excellent properties, including high thermal conductivity (130-200 Wm−1K−1) and low coefficient of thermal expansion (CTE) (4-4.5 ppmK−1). The combination of high thermal conductivity and low CTE gives aluminum nitride good thermal shock resistance. Furthermore, aluminum nitride has a flexural strength exceeding that of alumina and beryllia, exhibits a low hardness which enables it to be machined easily, and is stable at temperatures in excess of 900° C. in an oxidizing environment and up to 1600° C. in a reducing environment.
Despite the promise of aluminum nitride, application of thick-films on aluminum nitride or aluminum nitride-containing substrates is severely limited by the lack of compatible thick-film paste compositions which adhere sufficiently to such materials. In order to adhere metal conductors to aluminum nitride substrates, the use of thick film technology is typically used, which adheres the conductor via a thin reactive layer (oxide film) formed between the metal and substrate by introducing the metal in atomic form to the surface of the ceramic substrate so that the metal, which is extremely active chemically, bonds with the excess oxygen that exists in the surface of the substrate.
There is, therefore, a need for new metal electroconductive paste compositions that exhibit high stability and excellent adhesion properties to an underlying aluminum nitride substrate.